Simple Telephone Circuit

What are you using for microphones? A 12V battery with a 200-300 ohm series resistor to one wire in each "line" with the other wires grounded will make a phone work, but there's more to a phone than any old microphone and any old speaker.

A "regular" (i.e. dynamic) mic connected to a speaker, with or without battery current, will sound weakly or not at all. A phone connection will work dandy with just a pair of phones from Radio Shack and the afore- mentioned 12V battery and resistor.

In real telephone exchanges of old they used inductors (actually relay coils) where you're using resistors. Inductors allow DC current to flow to power the carbon microphones whilst blocking AC. This means all the AC power is transferred to the receiver. Your resistors will be absorbing some of the available power. You can slo connect a battery, carbon microphone and receiver in series without resistors or inductors.

You might find my home made automatic telephone exchange interesting:

Thanks for taking the time to prepare this site. It's wonderful to read through.

Jon

It seems to make more sense to me that all the devices share the same current in a loop, so I believe series is preferred. That includes the microphones and high impedance speakers, I think, if you can use carbon granule type microphones (resistive) or design an appropriate circuit to adapt readily available microphones so that they modulate the current. Also, doing it this way allows separate loops to be tapped off the same battery terminals (assuming it has the capacity) without interfering much with each other. It seems to get more complex, if you don't do that.

What type of microphone and speaker (8 ohm?) is involved? Also, I think you need to learn about inductors/transformers for this to work really well (or else design active circuits for it.)

Separate current loops may work reasonably well.

Also take a look at this:

It may be suggestive of something or some other web site to find.

Excellent project, by the way.

Jon

We did something similar when I was a kid, we used some old telephone handsets that had carbon microphones and wired them in series with the ear piece. With 3 volts in series it worked pretty good. No patch panel though. We had 3 "forts" out in the pasture and all the units were just connected in series. Beat the heck out of a string and tin can.

Jimmie

Hi All,

Thank you all for your responses from before (summary of the project is above). In further experimenting with this, we've found that while things work to a degree, the volume is really low. We've been playing with a variety of computer microphones, cheap headsets, and telephone handsets. It may be it might work better with very high-gain microphones, but I haven't really gone on a microphone hunt to experiment with this.

So what I'm thinking now is that perhaps putting a simple transistor amplifier in the circuit would give us a reasonable volume. I Googling around, I found the following "microphone preamplifier" that uses only one transistor, nice and simple:

My question: given that I'm hooking the microphone and speaker together at each end of the telephone circuit for a two-wire (signal and ground) connection, what's the appropriate way to put the amplifier in the circuit, given the amplifier has an input/output, and the telephone wire is a two-way signal?

Or do I want two amplifiers, one for each incoming line, and then how should those connect together?

Thank you in advance for any advice!

Tim

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Your design of the 8-lines-telephone-exchange is very interesting to read. I have seen relay exchanges using mechanical gears and levers and uniselectors. What we call a strawger type or "Step by step" exchange. Also cross-bar exchanges using relays arranged in horizontal and vertical bars for switching. And finally the SPC (stored program control) exchanges by Ericsson AXE and NEC NEAX type exchanges. The early Ericsson AXE exchanges used reed relays for connecting the subscribers' phones and the later ones are totally digital using ADC and mutiplexing techniques.

But I have never seen an exchange using CMOS crosspoint switch. Are these CMOS xpoints something like the 4016 or 4066 gates? How many of these chips are required to make a 4-line-PABX? Have you designed one with these CMOS chips?

Allen

Though I've never constructed a telephone set before, I think your mic amp might work with your circuit. Just arrange the receiver and mic like the schematic in the EE-8 phone in the link supplied by Jon. You may use the current feed from the line through a coil to power your transistor circuit or you may use button cell to power the transistor and just connect the output in series with the receiver. You need to install one circuit per mic per phone set.

The old POTS are very simple where there are no active components. There is a hybrid transformer to step up the transmitter level as well as to minimise the side-tone. Modern telephones are quite complex and there are normally 2 chips, one for the transmission bridge and the other for the keyboard encorder and DTMF. SOme sets have only one LSI SMT chip and even LCD panels.

Allen.

A parallel system is best as it allows you to use a "common battery" power source.

See below

Put an audio inductor in series with the battery. It could be the primary of an old transistor radio output transformer (don't use the secondary winding). The polarity isn't important to your application. The inductor will prevent the audio signal of the phones from being attenuated by the low dc resistance of the battery.

Connect each of the phones in parallel with the common battery/ inductor. They will communicate with good volume.

Have fun.